The present invention relates generally to ceramic arc discharge lamps and more particularly to a discharge lamp with an end zone having reduced wall thickness and a dose comprising sodium, thallium, calcium, and lanthanum, generally in the form of their halides, which is suitable for lamps having a wattage in the range of 15-100 watts.
Discharge lamps produce light by ionizing a fill material, such as a mixture of metal halide and mercury in an inert gas, such as argon, with an arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge chamber, which maintains the pressure of the energized fill material and allows the emitted light to pass through. The fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being vaporized and excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, including color temperatures, color rendering, and luminous efficiency.
Conventionally, the discharge chamber in a discharge lamp was formed from a vitreous material such as fused quartz, which was shaped into desired chamber geometries after being heated to a softened state. Fused quartz, however, has certain disadvantages, which arise from its reactive properties at high operating temperatures. For example, in a quartz lamp, at temperatures greater than about 950-1000° C., the halide filling reacts with the glass to produce silicates and silicon halide, which results in depletion of the fill constituents. Elevated temperatures also cause sodium to permeate through the quartz wall, which causes depletion of the fill. Both depletions cause color shift over time, which reduces the useful lifetime of the lamp. Color rendition, as measured by the color rendering index (CRI or Ra) tends to be moderate in existing quartz metal halide (QMH) lamps, typically in the range of 65-70 CRI, with moderate lumen maintenance, typically 65-70%, and moderate to high efficacies of 100-150 lumens per watt (LPW). U.S. Pat. Nos. 3,786,297 and 3,798,487 disclose quartz lamps which use high concentrations of cerium iodide in the fill to achieve relatively high efficiencies of 130 LPW at the expense of the CRI. These lamps are limited in performance by the maximum wall temperature achievable in the quartz arctube.
Ceramic discharge chambers were developed to operate at higher temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. In general, CMH lamps are operated on an AC voltage supply source with a frequency of 50 or 60 Hz, if operated on an electromagnetic ballast, or higher if operated on an electronic ballast. The discharge is extinguished, and subsequently re-ignited in the lamp, upon each polarity change in the supply voltage.
One problem with such lamps is that the light output deviates from that of “white” light. One way to measure this is as the difference in chromaticity of the lamp's color point, on the y axis (ccy) from that of the standard black body curve plotted on a CIE (Commission Internationale de l'Eclairage) 1931 chromaticity diagram in which the chromaticity coordinates represent relative strengths of two of the three primary colors, denoted by x and y. This chromaticity difference is referred to herein as Dccy. The black body curve (or Planckian locus) represents the color points on the CIE chromaticity diagram traversed by an incandescent object as its temperature is raised and occupies the central white region. Two lamps whose x, y coordinates fall one above the black body curve and one below could have the same correlated color temperature (CCT) while having a different hue. For many applications, it is desirable to have light with virtually no hue, e.g., without a greenish or reddish tint.
The properties of high intensity discharge lamps operated at high temperatures tend to suffer. Ceramics operated at high temperature degrade in their mechanical strength, and consequently the lamps may not withstand the stresses on the ceramic that are present during lamp operation. This leads to premature lamp failure or poor reliability. CRI, lower CCT and dCCy close to the black body locus are often all desired, thus lamp lumen maintenance generally has to be sacrificed.
The exemplary embodiment provides a ceramic metal halide lamp capable of emitting light which is close to the black body curve, which overcomes the above-referenced problems and others.